The present invention relates generally to optical processing, and more specifically, to improved optical processing using an apodization mask.
Apodization masks are employed to softly shape an optical beam in optical instruments, such as, an interferometer. One purpose of using such masks is to prevent diffraction rings from being generated due to hard edge clipping. Hard edge clipping might create interfering signals that can degrade the resulting metrology measurement. FIG. 1 is a simplified schematic diagram illustrating a portion of a pattern used in a conventional apodization mask.
Normal apodization masks are restricted to symmetrical optical density patterns which are created by circular deposition of an opaque material, such as, chrome. The required radial variation in optical density also results in a varying optical path length difference, which can amount to hundreds of nanometers. With interferometers operating in the picometer range, this variation becomes unacceptable.
Conventional apodization masks present at least a number of problems for use with advanced interferometers. For example, interferometers may not necessarily have rotationally symmetric beam shapes. As a result, uniquely shaped mask patterns may be required. Such uniquely shaped mask patterns, however, cannot be fabricated by conventional rotationally symmetric deposition technology. In addition, in very highly resolving interferometric instruments, the varying density is also seen as a varying thickness or optical path length difference and results in an undesirable curved wavefront. Such an undesirable curved wavefront may introduce errors in applications requiring super-precise metrology. Moreover, conventional chemical/ion beam vapor deposition may not be suitable for fabricating a non-rotationally symmetric mask. Furthermore, some apodization masks are created using zig-zap structures which are only partially effective and difficult to manufacture.
Hence, it would be desirable to provide a method and system that may be used to fabricate an improved apodization mask for optical processing.